Creating a function dynamically at run-time

问题:

It probably isn't even possible to do this, but I will ask anyway. Is it possible to create a function that receives a string and then uses it as a right side argument for the goes to operator (=>) used in lambda?

Actually, what I want to do is to be able to redefine an specific method of a specific class during runtime. I want to be write down a function with the program running and attaching it to a delegate. Is it possible?

回答1:

The easiest way to do it is probably DLINQ as TcKs suggested.

The fastest (I believe, in 3.5) is to create a DynamicMethod. Its also the scariest method as well. You're essentially building a method using IL, which has about the same feel as writing code in machine language.

I needed to do this to dynamically attach event handlers in some thing or another (well, I didn't need to do it, I just wanted to make unit testing events easier). It seemed a bit daunting at the time because I don't know crap about IL, but I figured out a simple way to accomplish this.

What you do is create a method that does exactly what you want. The more compact the better. I'd provide an example if I could figure out exactly what you're trying to do. You write this method in a class within a DLL project and compile it in release mode. Then you open the DLL in Reflector and disassemble your method. Reflector gives you the option of what language you wish to disassemble to--select IL. You now have the exact calls you need to add to your dynamic method. Just follow the example on MSDN, switching out the example's IL for your reflected methods' code.

Dynamic methods, once constructed, invoke at about the same speed as compiled methods (saw a test where dynamic methods could be called in ~20ms where reflection took over 200ms).

回答2:

You have several ways how to do it:

• dynamically create lambda expression (look at Dynamic LINQ: Part 1)
• dynamically create CodeDom model and compile it at run-time (look at CodeDom.Compiler namespace
• dynamically create C#/VB.NET source code and compile it at run-time (look at CSharpCodeProvider and VBCodeProvider classes )
• dynamically create object model, which can do same things like the method (look at Strategy Design Pattern

回答3:

Your question is pretty unclear, but you can certainly use expression trees to create delegates dynamically at execution time. (There are other ways of doing it such as CodeDOM, but expression trees are handier if they do all you need. There are significant restrictions to what you can do, however.)

It's often easier to use a lambda expression with some captured variables though.

For example, to create a function which will add the specified amount to any integer, you could write:

static Func<int, int> CreateAdder(int amountToAdd)
{
    return x => x + amountToAdd;
}

...
var adder = CreateAdder(10);
Console.WriteLine(adder(5)); // Prints 15

If this doesn't help, please clarify your question.

回答4:

Not that I am recommending this over the other better options, but there is a 7^th method and thats to use AssemblyBuilder,ModuleBuilder,TypeBuilder, and MethodBuilder in the System.Reflection.Emit namespace to create a dynamic assembly. This is the same similar voodoo as using DynamicMethod.

For example you could use these to, at runtime, create a proxy class for a type and override virtual methods on that type.

To get you started here is some code...

using System;
using System.Reflection;
using System.Reflection.Emit;

var myAssembly = AppDomain.CurrentDomain.DefineDynamicAssembly(
        new AssemblyName("Test"), AssemblyBuilderAccess.RunAndSave);
var myModule = myAssembly.DefineDynamicModule("Test.dll");
var myType = myModule.DefineType("ProxyType", TypeAttributes.Public | TypeAttributes.Class,
                        typeof(TypeToSeverelyModifyInAnUglyWayButItsNecessary));
var myMethod = myType.DefineMethod("MethodNameToOverride", 
                        MethodAttributes.HideBySig | MethodAttributes.Public,
                        typeof(void),Type.EmptyTypes);
var myIlGenerator = myMethod.GetILGenerator();
myIlGenerator.Emit(OpCodes.Ret);
var type = myType.CreateType();

回答5:

You should check to see if your problem can be solved with simple polymorphism first. Unless you're defining abstract interoperability to another language or editing the compiler, trying to change a method at runtime is probably the wrong solution to the problem.

回答6:

If you declare the method as virtual, you might be able to use Castle's DynamicProxy to substitute a dynamically-generated (with one of the other answer's methods) implementation at runtime:

Castle DynamicProxy is a library for generating lightweight .NET proxies on the fly at runtime. Proxy objects allow calls to members of an object to be intercepted without modifying the code of the class. Both classes and interfaces can be proxied, however only virtual members can be intercepted.

回答7:

The second paragraph in your question suggests that really what you might be after is straightforward IOC (Inversion of Control)

Instead of declaring an instance of your class, you declare an instance of an interface and based on whatever condition you choose, you use the particular overriding class with the correct method in it. Hope that makes sense.